By growth inhibiting mouse L-cell monolayers with Actinomycin, many cytotoxins can be found in supernatants of mitogen-activated lymphocytes. One toxin, termed beta-LT, is 40,000 molecular weight, relatively unstable (60% loss in activity in 24 hours at 4 degrees centrigrade), and is released in relatively large amounts during the frist 24 hours in culture. Another set of toxins, called alpha-toxin (alpha-LT), are 80,000 molecular weight. Their rate of release increases greatly after 24 hours. The in vivo junction of these toxins is unknown. Three facts suggest a suppressor role for alpha-LT. The toxic activity of alpha-LT is inhibited by single strand DNA. Also, alpha-LT inhibits hen oviduct RNA polymerase II and DNA polymerase. Lastly, purification of alpha-LT almost to the point of homogeneity still maintains the same ratio of biologic activities of mouse L-cell cytotoxicity and inhibition of human lymphocyte DNA synthesis. An attractive operational premise is that alpha-LT mediates the effect of suppressor T-cells, and beta-LT, that of cytotoxic T-cells. Current work is relating the different toxins, which can be differentiated by size, ionic exchange, cellulose binding characteristics and by heterologous antisera, to in vitro measures of cell-mediated cytotoxicity and suppression.